The Imprint of Relativistic Particles on the Anisotropies of the Stochastic Gravitational-Wave Background

TL;DR

Relativistic particles in the early Universe influence the anisotropies of the stochastic gravitational-wave background, providing a new way to probe particle physics and cosmology through future GW observations.

Contribution

This paper demonstrates how relativistic particles affect SGWB anisotropies, linking particle physics to gravitational-wave cosmology in a novel way.

Findings

Relativistic particles alter the angular power spectrum of SGWB.

Changes in relativistic particle content affect Sachs-Wolfe and ISW contributions.

Future GW detectors can test these effects through large-angle anisotropy measurements.

Abstract

The Stochastic Gravitational-Wave Background (SGWB) is expected to be a key observable for Gravitational-Wave (GW) interferometry. Its detection will open a new window on early Universe cosmology, on the astrophysics of compact objects and, as shown in this Letter, on the particle physics content of the Universe. In this Letter we show that, besides their effects on the Cosmic Microwave Background (CMB) and on Large Scale Structure (LSS), relativistic particles in the early Universe leave a clear imprint on the anisotropies of the SGWB. In particular we show that a change in the number of decoupled relativistic particles shifts the angular power spectrum of the SGWB, as both the Sachs-Wolfe (SW) and the Integrated Sachs-Wolfe (ISW) terms are affected. Being very large-angle effects, these lead to new testable predictions for future GW interferometers.